Threshold shear strain is one of the basic parameters of soil during cyclic loading condition. The threshold shear strain should be determined for evaluating pore water pressure generation due to earthquake, vibrating machine and all the cyclic loading which apply permanently or temporary on the soil. According to many scientific researches, the threshold shear strain is the strain, which there is no pore water pressure generation at the saturated soils and no volume changes at the dry conditions during cyclic loading. By increasing the shear strain more than threshold shear strain ( ), pore pressure increase is noticeable at the saturated soil and the soil microstructure starts to change. In this study, the effects of plastic and non-plastic fines on the threshold shear strain of saturated sands were evaluated by performing a number of cyclic triaxial tests with strain control method at small shear strain. All the specimens contained 0%, 10%, 20% and 30% plastic and non-plastic fines prepared by wet tamping undercompaction method. As a result, by increasing plastic fines more than 20%, threshold shear strain increased significantly. Moreover, by increasing non-plastic fines, threshold shear strain fluctuated. It first increased and then decreased.